The present invention relates to a method of treating pulp and to an apparatus for applying the method. The primary object of the invention is to develop a method and an apparatus for treating pulp at a temperature which as precisely as possible meets the requirements of the treatment, and thereby to optimize the process with respect to also the temperature. A particular object of the invention is a method of bleaching pulp with a sequence using chlorine dioxide, in as environmentally friendly a way as possible. The method of the invention is especially suitable for small-scale rebuilds of existing pulp mills, which are based on use of elemental chlorine and chlorine dioxide. A second object of the invention is an elemental chlorine free bleaching sequence, which is suitable for places where chlorine dioxide is, however, desired to be used. Use of chlorine dioxide is, however, minimized in the sequence by arranging for optimal process conditions for chlorine dioxide in the dioxide bleaching, whereby the AOX number of effluents may be maintained very at a very low level. The invention also relates to an apparatus used, e.g., in bleaching, which apparatus enables a significant reduction of steam consumption at the mill. Especially, reduction in use of high-pressure steam is concerned because the mills often have more than enough low-pressure steam, but in some cases the mill is not even capable of producing as much high-pressure steam as the heating requires. The invention also relates to treatments which are effected with other bleaching and treatment chemicals, and in which the treatment temperature needs to be changed between different stages.
Pulp mills are nowadays trying to get rid of use of elemental chlorine and partly also of chlorine dioxide. Reasons for this are both environmental protection and market factors. Drawbacks caused by elemental chlorine are clearly noticeable malodorous gaseous emissions as well as liquid discharges from pulp mills to water-courses. Liquid chlorine dioxide is not so much of a nuisance as for its odour, but it has drawbacks which mainly affect watercourses. However, when these chlorine chemicals are compared with each other by means of the AOX number, which indicates their load on the water-courses, it can be seen that the harmfulness of elemental chlorine is multiple in comparison with that of chlorine dioxide. The AOX number of chlorine is about 4 to 7 and that of chlorine dioxide about 1 to 1.5 or even slightly below 1.
However, sequences using chlorine dioxide are still favoured and they are feasible as for environmental aspects also. There are many reasons for that. In comparison with other chemicals, the price for chlorine dioxide is very competitive; it is today approx. half of the price for, e.g., competing hydrogen peroxide or its various derivatives. Moreover, the strength and brightness values of the pulp obtained by dioxide bleaching are high. In fact, they are at least of the same standard as the values obtained by using peroxide, with the same chemical consumption (kg/adt).
An object of the invention is to apply, in an economical. and pro-environmental manner, at least one chlorine dioxide stage (D) to a bleaching sequence, which uses modern bleaching chemicals such as ozone (Z) or peroxide (P), in other words, hydrogen peroxide or some other chemical behaving like a hydrogen peroxide in bleaching. Instead of using chlorine dioxide as the main bleaching chemical as it has been used earlier, its role has been changed in our invention. Here it is in the first place used as an additional or auxiliary chemical for activating and subjecting pulp to the influence of the main chemical. In this case, when chlorine dioxide is used together with peroxide and ozone in accordance with the present invention, the AOX number of chlorine dioxide may be even less than 0.5. Furthermore, the filtrates from the chlorine dioxide stage may be treated together with the filtrates from the chelating stage in a separate evaporating unit, so chlorine discharges as such are prevented from escaping from the dioxide stage to effluents. This procedure already prevents chlorine compounds from ending up in the mill effluents.
The amount of chlorine dioxide (D) actually used, e.g., in sequences ZD-EOP-ZD-D or ZD-EOP-DQ-P.sub.O remains very low in our invention. Chlorine dioxide is primarily used as an additional chemical in ozone bleaching (Z) and peroxide bleaching (P), to improve the process economy and pulp quality. A chlorine dioxide dosage may be even as low as 1 to 30 kg chlorine dioxide calculated as active chlorine per pulp ton. Normally, the need may be 10 to 20 kg chlorine dioxide per pulp ton. In combined stages DQ and ZQ (Q denotes metal removal in general) use of peroxide is decreased because metal removal done with, e.g., these stages, prior to peroxide is highly efficient. ZD as well as DZ reduces use of chlorine dioxide because each kilogram of ozone decreases the need for use of chlorine dioxide by 1-4 kg/adt. EOP is an alkali stage where a peroxide dosage is 0 to 10 kg/adt and P.sub.O is a peroxide stage where the peroxide dosage is 5 to 40 kg/adt. Both stages also use oxygen, and the dosage is 0 to 15 kg/adt, preferably 3 to 8 kg/adt.
Use of combined stages ZQ and ZD is disclosed, e.g., in A. AHLSTROM CORPORATION's U.S. patent application No. 380,486. Both DQ and AQ are disclosed in A. AHLSTROM CORPORATION's patent applications FI 950749, FI 953343, and SE 9502078. DQ and AQ stages are advantageous prior to the peroxide stage.
Chlorine dioxide bleaching (D) is a well-known and widely used bleaching method. In conventional chlorine dioxide bleaching, the temperature is usually 70 to 90.degree.C. and the dosage of chlorine dioxide is approx. 10 to 30 kg per pulp ton. Sequences used are, for example, D.sub.0 ED.sub.1 ED.sub.2 and OD.sub.0 ED.sub.1 ED.sub.2 and various modifications of these. The treatment time in the D.sub.0 stage is normally shorter than in other D stages; it is, for example, 30 to 90 minutes. The pH of pulp lowers to a range of 1 to 3 at the end of the D.sub.0 stage. In the D.sub.1 and D.sub.2 stages, the treatment time is 2 to 3 h and the pH is slightly higher than in the D.sub.0. Earlier, it was common for the pulp entering the first stage D.sub.0 that it contained plenty of lignin, which was quickly removed at the beginning of bleaching. Elemental chlorine was, and still is, generally used as an additional chemical in the D.sub.0 stage for removing lignin. The conditions in the D.sub.0 stage had been optimized for lignin removal.
When switching over to pro-environmental cooking and bleaching techniques, an object is to have a kappa number of pulp which is below 20, preferably below 12, with efficient cooking and subsequent oxygen delignification. After efficient cooking and oxygen delignification, use of earlier known D.sub.0 stages has continued by force of habit in prior art installations, as it has been assumed that the pulp still contains relatively much lignin. However, this is not wise, as explained in the following. Namely, it has been established that when the kappa number is this low, the pulp no longer contains much lignin. In other words, the kappa number is not so much dependent on lignin but mainly on carbohydrate compounds, for example, different acids. For removal of these, there are better and more efficient ways than the conventional D.sub.0 stage.
The conventional D.sub.0 stage is not optimal if the intention is to remove something else than lignin.
A prior art method of decreasing the kappa number of pulp by 1-9 units is to treat the pulp with acid (A). At A stages, the pH is within the range 1 to 6, preferably 2 to 5, and most preferably 3 to 4 when the explicit intention is to decrease the kappa number. The intention of these treatments is, e.g, to decrease the kappa number of pulp prior to actual bleaching stages. By raising the temperature, it is possible to accelerate the lignin removal reaction. An advantageous temperature is 80 to 140.degree. C., most preferably 90 to 110.degree. C. The most suitable temperature has often been considered to be 90 to 100.degree. C., at which temperature a reaction time of 60 to 180 minutes, preferably 60 to 120 minutes, provides the decrease in the kappa number which is obtainable in the first place with acid and/or chelation treatment. The kappa number of hardwood decreases more than that of softwood.
Normally, the kappa number of softwood decreases by 1-3 units and the kappa number of hardwood by 3 to 6 units in an acid treatment.
An object of the present invention is to disclose how the D stage, especially the D.sub.0 stage, may be intensified by maintaining in the D stage conditions which are favourable to the kappa number reduction, and chlorine dioxide consumption be simultaneously decreased.
The chlorine dioxide stage is usually run at a temperature of approx. 70.degree. C., the treatment time at the D.sub.0 stage being 0.5 to 2 hours and at the D.sub.1 and D.sub.2 stages 2 to 3 h. It has been tried to avoid higher temperatures because a low final pH of 1 to 3 at the D stage, when combined with a high temperature and a long treatment time, damages the strength properties of fibers. It is, however, possible to raise the temperature and extend the duration of the D stage, especially of the D.sub.0 stage if it is seen to that the pH is maintained over 2, preferably over 3. The chemical reactions reducing the kappa number of the A or Q stage may be effected at the D stage by seeing to that, during the D stage the pH is within the range of 2 to 5, preferably 3 to 4, and the temperature within the range of 80 to 130.degree. C., preferably 90 to 110.degree. C. Part of the treatment time of the D stage the pH may be something else, and naturally this is the case, too, because reacting of chlorine dioxide produces acids. It is, however, essential the treatment time in the pH range of 2 to 5, preferably 3 to 4, is long enough, i.e., over 40 minutes, but preferably longer, usually over 60 minutes. It is also essential that the pH is not below 2 for too long a time since this damages fibers, as was stated above. It is also essential that the method is applied to a pulp in which the kappa number has been decreased to a value below 20, preferably below 12, by cooking and delignification, so that also other matter than lignin may be removed at the dioxide stage.
Another object of the invention is to provide an apparatus for meeting the technical requirements of the method. In other words, such an apparatus has neither been used nor even existed by which pulp could have been heated or cooled to a desired temperature and through which pulp could have been fed to a treatment tower at the same time equalizing the pulp temperature. This object of the invention is achieved by an apparatus which comprises an indirect heat exchanger, mixing/feed means, and a treatment tower.
A still another object of the invention is, on the one hand, to strive for decreasing the energy consumption of the mill, and on the other hand, to make it possible to connect treatments done at various temperatures in an economical manner one after another in order to form a bleaching sequence. For example, use of chlorine dioxide together with, e.g, peroxide, the latter being very popular today, is problematic because bleaching with chlorine dioxide usually takes place at a slightly lower temperature than bleaching at the efficient peroxide stage. In other words, for the chlorine dioxide stage subsequent to the peroxide stage, pulp should be cooled to some extent and, correspondingly, for the peroxide stage subsequent to the chlorine dioxide stage it should be heated. If a well-known prior art way of heating, i.e., direct steam heating were used, there would be a considerable increase in the steam consumption of the mill, which would also add to the costs remarkably.
The same problem can be seen also in sequences using ozone because ozone is most usually used at a temperature of approx. 50 to 80.degree. C., but higher temperatures (80 to 110.degree. C.) are also possible.
It is previously known to use high-pressure steam for the above-mentioned purposes, i.e., for heating pulp for bleaching, by which high-pressure steam pulp is heated directly. The equipment according to prior art is comprised of a so-called MC pump, which feeds pulp to the bleaching stage (the process most usually takes place at a consistency of 10 to 15%, which is a so-called medium consistency or an MC range), steam feeding means, mixer, reaction tower, means discharging pulp from the tower, blow tank where pulp is passed from the tower, pulp discharge pump, and condenser for discharged steam. As the above list of equipment implies, the bleaching process advances as follows: the pulp is fed by a pump to the steam feeding means, where the pulp temperature is raised to a desired level by feeding high-pressure steam directly into the pulp. After adding the steam, the pulp is conveyed to the mixer, which, besides equalizing potential temperature differences brought about the addition of steam, also mixes a desired bleaching chemical/bleaching chemicals or other treatment chemicals, such as chelating agent, acid, or alkali with the pulp. From the mixer, the pulp is further conveyed to the reaction tower, where the bleaching/treatment reaction itself is allowed to take place. For example, in peroxide bleaching, the temperature in the tower is maintained at approx. 100.degree. C.; even 130.degree. C. temperatures have sometimes been suggested, and the pressure in the lower part of the tower is approx. 10 to 8 bar and in the upper part thereof approx. 5 to 3 bar. The pulp is discharged from the tower by the discharge means to the blow tank, where the steam still present in the pulp is separated therefrom into the upper part of the blow tank. From the blow tank the pulp is discharged by the discharge pump. The steam separated to the upper part of the blow tank is further conveyed to the condenser, where the heat still present in the steam is recovered, thereby generating condensation water.
The process described hereinabove, however, involves some drawbacks:
Firstly, a great portion of steam is condensed in the pulp, so the pulp consistency is no longer the same it was when the pulp left the pump. PA1 Secondly, the pulp pressure in the steam feeding means has to be limited to approx. 9-10 bar because there is no steam available which would be at a higher pressure. Thereby, also the process pressure in the reaction tower is limited to the above-mentioned value. PA1 Thirdly, for recovering heat and conveying pulp to the next process stage, a large combination including a blow tank, pump, and condenser, is needed. PA1 Fourthly, the highest temperature of the condenser is 100.degree. C. because the pressure is lowered to the same level as the outside air pressure. PA1 Fifthly, the condensation water from the condenser is dirty because it contains residues from both bleaching chemicals and bleaching reaction products. PA1 the pulp is cooked and, if necessary, oxygen delignified-to a kappa number of under 20, preferably under 12; PA1 the pulp is bleached in a chlorine dioxide stage the temperature of which is 90 to 110.degree. C., preferably 95 to 100.degree. C., so that the pH is within the range of 2 to 5, preferably 3 to 4, for over 40 minutes, preferably 60 to 120 minutes while the chlorine dioxide dosage is over 5 kg active chlorine per pulp ton. PA1 the pulp is fed with a high-consistency pump within a consistency range of 8 to 20% to an indirect heating means which is provided with heat transfer surfaces, PA1 after the heat exchanger, the pulp is conveyed to a mixing device, which is preferably fluidizing, and this mixing device or a device preceding thereof is supplied with bleaching chemical, which is preferably chlorine dioxide, PA1 the pulp is transferred to a bleaching tower, which is preferably pressurized, the pressure being 0.1 to 15 bar overpressure, in which tower the pulp is bleached, the treatment time being 40 to 180 minutes, and PA1 the pulp is discharged from the tower preferably with a fluidizing discharge means, which may be supplied with additional chemicals.
Besides solving the above-mentioned problems involved in direct heat exchange in a very elegant manner, the present invention also makes some bleaching sequences which are already very practicable and efficient and which use chlorine dioxide, chelation, acid treatment, peroxide peracetic acid and/or ozone or various combinations of these, still more attractive.
A characterizing feature of the method in accordance with a preferred embodiment of the invention is that the chlorine dioxide stage according to the invention is the first chlorine dioxide stage of the bleaching sequence, i.e., a so-called D.sub.0 stage. A suitable chlorine dioxide dosage for this kind of stage is 1 to 40 kg chlorine dioxide calculated as active chlorine per pulp ton. Preferably, the dosage is 5 to 30 kg, and most preferably 5 to 20 kg per pulp ton.
It is a characterizing feature of a second preferred embodiment of the invention that
In accordance with a third preferred embodiment, a chlorine dioxide solution is preheated with heat transfer surfaces, which are preferably made of titanium and disposed in either the bleaching tower or the outlet duct for pulp.
It is a characterizing feature of the method in accordance with a fourth preferred embodiment of the invention that
Other characterizing features of the method and the apparatus according to the invention will become apparent from the enclosed claims.
Laboratory tests indicate that in bleaching of softwood pulp with chlorine dioxide, the need for chlorine dioxide in a D.sub.0 stage according to the above described invention is over 10% less than in a conventional D.sub.0 stage. With hardwood pulp, the difference is still more significant. In other words, the need for chlorine dioxide with hardwood in the D.sub.0 stage of the invention is over 25% less than in a conventional D.sub.0 stage. The difference is caused by the fact that when bleaching with chlorine dioxide is effected at too low a temperature or at an unsuitable pH value, the only bleaching reactions are reactions of chlorine dioxide. When the pH value is correct, preferably 3 to 5, and the temperature sufficiently high, i.e., 80 to 110.degree. C., hot and acid conditions alone perform part of the bleaching.